Wireless terminals are enabled to communicate wirelessly in a radio communications system, sometimes also referred to as a radio communications network, a mobile communication system, a wireless communications network, a wireless communication system, a cellular radio system or a cellular system. The communication may be performed via a radio channel, e.g. between two wireless terminals, between a wireless terminal and a regular telephone and/or between a wireless terminal and a server via a Radio Access Network (RAN) and possibly one or more core networks, comprised within the wireless communications network.
A wireless terminal, sometimes referred to as a user terminal or a User Equipment (UE), is a mobile terminal by which a subscriber can access services offered by an operator's core network.
A cellular radio system covers a geographical area which is divided into cell areas, wherein each cell area is served by a network node such as a base station. In 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE), base stations may be referred to as eNodeBs or eNBs. A cell is the geographical area where radio coverage is provided by the base station at a base station site.
One base station, situated on the base station site, may serve one or several cells. Further, each base station may support one or several communication technologies. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
The base stations may be of different classes such as e.g. macro base stations, macro eNodeBs, home eNodeBs or pico base stations, based on transmission power and thereby also cell size.
During the past few years, wireless operators have offered mobile broadband services based on Wideband Code Division Multiple Access/High Speed Packet Access (WCDMA/HSPA). End user performance requirements have also increased, fuelled by new devices designed for data applications. The large uptake of mobile broadband has resulted in heavy traffic volumes that need to be handled by the HSPA networks, which have grown significantly. Therefore, techniques that allow operators to manage their spectrum resources more efficiently are of great importance.
It is possible to improve the downlink performance by introducing support for techniques such as 4 branch Multiple Input Multiple Output (MIMO), multiflow communication, multi carrier deployment, etc. Improvements in spectral efficiency per link are approaching theoretical limits. As a result, the next generation technology tends to focus on improving the spectral efficiency per unit area. Additional features for High Speed Downlink Packet Access (HSDPA) should then provide a uniform user experience to users anywhere inside a cell by changing the topology of traditional networks. Currently 3GPP has been working on this aspect using heterogeneous networks.
A homogeneous network is a network of base stations, e.g. Node Bs, in a planned layout and a collection of wireless terminals. In the homogeneous network all base stations have similar transmit power levels, antenna patterns, receiver noise floors, and similar backhaul connectivity to the data network. Moreover, all base stations offer unrestricted access to wireless terminals in the network, and serve roughly the same number of wireless terminals. Current wireless systems that come under this category include Global System for Mobile communications (GSM), WCDMA, HSDPA, LTE, and Worldwide Interoperability for Microwave Access (WiMax).
In a heterogeneous network (HetNet), in addition to the planned or regular placement of macro base stations, several pico and/or femto and/or relay base stations are deployed as illustrated in FIG. 1a. The power transmitted by these pico and/or femto and/or relay base stations, being up to 2 W, is relatively small compared to that of the macro base stations, up to 40 W. These Low Power Nodes (LPN) are typically deployed to eliminate coverage holes in the homogeneous network using macro base stations only. The LPNs may improve capacity in hot-spots. Due to their low transmit power and small physical size, the pico/femto/relay base stations may offer flexible site acquisitions.
Heterogeneous networks may be divided into two deployment categories: co-channel deployment and soft cell deployment. The latter is also referred to as shared or combined cell deployment. In the co-channel deployment, an LPN has a cell identifier different from that of the macro node, i.e. the LPNs create different cells. In the soft cell deployment, each LPN has a cell identifier which is the same as that of the macro node.
FIG. 1b illustrates an example of a heterogeneous network where the low power nodes create different cells, which is an example of the co-channel deployment. Simulations indicate that significant gains in the system throughput as well as cell edge user throughput may be realized through the co-channel deployment. One reason for the improved throughput is that the co-channel deployment provides opportunities for load balancing. In a heavy data traffic scenario, the load in the macro cell may be shared between the macro node and the low power nodes. Also, users with low Signal-to-Noise-Ratio (SINR) may be served by strategically located LPNs. In short, the LPNs may provide resources to serve users and thereby increase average user throughput of the network.
However, since each LPN creates a different cell, one disadvantage of the co-channel deployment is that a soft handover is necessary when a wireless terminal moves from one LPN to the macro node or to another LPN. As a result, a higher layer, e.g. above physical layer, signaling is necessary to perform the handover.
FIG. 1c illustrates a heterogeneous network with a combined cell deployment. As indicated, the LPNs are part of the macro cell in this deployment. As such, the combined cell deployment may avoid the frequent soft handovers, and hence, may avoid the higher layer signaling.
In a combined cell deployment, all the nodes may be coupled to a central node, e.g. to the macro node, via high speed data link as shown in FIG. 1d. In the figure, the controlling central node in the combined cell may take responsibility for collecting operational statistics information of network environment measurements. The decision of which nodes to transmit to a specific wireless terminal may be made by the controlling central node based on the information provided by the wireless terminal. The cooperation among various nodes is instructed by the controlling central node and implemented in a centralized way.
Even though huge gains in terms of average sector throughput may be achieved with the introduction of LPNs, the interference structure becomes more complex in heterogeneous networks. For example, when a UE, such as a wireless terminal, is connected to an LPN, individual UE link throughput may be impacted due to the interference of the macro node power.